An exhaust gas sensor is mounted in the exhaust of an internal combustion engine and defines an example of such a sensor. The signal of this sensor is processed in a control apparatus having a ground potential which can be offset with respect to the ground potential at the location where the exhaust gas probe is installed. The reason for this possible offset is presented below.
Currents flow between the vehicle battery and the internal combustion engine. These currents, for example, supply the ignition equipment or are generated by a generator for charging the battery. The corresponding current loop is closed via the chassis, engine and the ground connection between the engine and the negative terminal of the battery and has a finite resistance. This resistance leads, in combination with the fluctuating current flow, to a potential difference between engine ground, chassis ground and battery ground. This potential difference fluctuates in dependence upon current intensity. The potential difference identified in the following as ground offset is between chassis ground and engine ground and typically is -0.3 to 1 Volt.
The following is based upon a so-called chassis ground concept; that is, the ground potential of a control apparatus corresponds to the ground potential of the chassis. The control apparatus is used to control functions of the internal combustion engine such as fuel metering and ignition. The offset between this ground potential and the ground potential of an exhaust gas probe mounted in the exhaust gas line comes about because the exhaust gas probe is often not insulated electrically with respect to the exhaust gas line and therefore is to some extent burdened by the ground potential of the engine.
The signal line is connected to the control apparatus. In combination with this signal line, a ground offset having the order of magnitude described can greatly falsify the signal of the exhaust gas probe because this signal can have a magnitude in the range of -80 mV to approximately 1 Volt and therefore have the order of magnitude of the possible ground offset.
U.S. Pat. No. 4,177,770 discloses a circuit for eliminating the above-described ground offset. The ground potential of the voltage supply for differential amplifiers in the control apparatus is tied to the ground potential of the exhaust gas probe. This tie-in is utilized to compensate the ground offset between the ground of the control apparatus and the ground of the exhaust gas probe.
A further possibility for avoiding the above-described ground offset comprises electrically separating the exhaust gas probe ground from the ground of the engine and connecting the exhaust gas probe ground, electrically insulated, to the control apparatus.
Separate from the problem of the ground offset, it is further known to impress a defined offset voltage onto the signal of the exhaust gas probe via a conditioning circuit and to further process the resulting total voltage. The offset is selected so great that the resulting total voltage is positive even for negative output voltage of the exhaust gas probe. This makes the evaluation of negative probe voltages possible.
Furthermore, it is possible by means of an impressed offset to compensate negative ground offset with a differential amplifier.
Further separate from the problem of the ground offset, it is known to impress a counter voltage onto the signal of the exhaust gas probe. The counter voltage has an influence on the resulting total voltage which is dependent upon the temperature of the exhaust gas probe and therefore on its internal impedance. For cold probes, only the counter voltage operates on the probe signal.
Likewise separate from the problem of ground offset, exhaust gas signal conditionings taking place without defined offset voltage are known. In this case, the output signal of the conditioning circuit should be a linear function of the input voltage without offset. The proportionality between the exhaust gas probe signal and the output signal of the conditioning circuit obtained thereby, affords the advantage that the original exhaust gas probe signal can be regenerated or again acquired in a simple manner, principally by means of a voltage divider from the output signal of the conditioning circuit. In this way, the original lambda signal can be outcoupled, that is, a signal is obtained for use by a service station which indicates directly the oxygen content of the exhaust gas.